Cache objects

Cache objects

The 1-wire bus is slow. A single query takes at least a few milliseconds. Temperature measurements can take as long as 1000 milliseconds. Computer memory is much faster (microseconds). Even network communication is faster

There are three ways around the problem:

  1. Ask for the data early, and do other work while it's being obtained.
  2. Remember the value in your program, but also remember when the data is stale.
  3. Let OWFS cache the data, removing it when it gets beyond a specified age.
 

 Cache with objects

 Here is a picture of the OWFS cache. Held in memory as a Binary Tree (for quick searching). The cache grows dynamically, storing various types of 1-wire data.

Cache Object Descriptions

The types of stored objects include:

  • device presence (and bus)
  • directory lists
  • properties.

Device properties can be either

  • volatile items that change on their own (like measured temperature), or
  • stable values, like alarm limits or memory contents. 

Typically, we would time out the volatile properties more quickly. 

Cache item expiring

When a value gets too old, it expires. Trying to read the 22.BB02/temperature value will trigger another 1-wire query, and the cache will be updated with the new value. Your program doesn't need to worry about the details.

Uncached

At any point, you can request a direct 1-wire read by looking at the /uncached directory (e.g. /uncached/28.CC03/temperature). That new value wil be placed in the cache as well, perhaps updating an existing reading.